1. Field of the Invention
This invention relates to optical waveguide fibers and, in particular, to methods and apparatus for recoating portions of such fibers with a UV-curable resin.
2. Description of the Prior Art
As is well known in the art, optical waveguide fibers are normally coated at the time of manufacture with a synthetic resin, e.g., a UV-curable resin, which strengthens and protects the fiber during use. The thickness of this original coating is carefully controlled so as to give the finished fiber uniform transmission, strength and appearance characteristics.
Both during manufacturing and in the field, it is often necessary to join (splice) two pieces of optical waveguide fiber together. As presently practiced, the glass portions of the fiber are fused together first and then the fiber is recoated in the area of the splice. The present invention is directed to the recoating portion of the splicing process, and, in particular, to the problem of recoating the splice so that the diameter of the recoated portion of the fiber matches the diameter of the rest of the fiber.
Various techniques have been used in the past to recoat spliced optical waveguide fibers. One such technique has involved the use of heat shrinkable tubing applied over the splice. Although shrinkable tubing does protect the splice, this approach is far from ideal since it produces a finished fiber having a non-uniform diameter and having two different types of coatings, i.e., the original resin coating and the shrinkable tubing coating.
Another approach has involved potting the splice in a bath of resin and then curing the resin by, for example, exposing it to ultraviolet light. As is evident, this approach does not produce either a uniform fiber diameter or a smooth joint.
A recoating technique employing a split recoating mold, having a transparent upper half made of plastic and a metallic lower half, was reported by A. C. Hart and J. T. Krause in Applied Optics, Vol. 22, No. 11, June 1, 1983, pages 1731-1733. In accordance with this technique, the splice which is to be coated, along with coated portions of the fiber on either side of the splice, are centered in a cylindrical groove formed in the mold. UV-curable resin is introduced into the groove using a syringe attached to a port which leads to the groove. After the groove has been filled, the syringe is removed and ultraviolet light is shined through the transparent upper half of the mold to cure the resin. The diameter of the groove is made slightly larger than the original diameter of the fiber to allow air to escape from the mold as the resin is injected into the groove and to compensate for the shrinkage of the resin upon curing.
As with the other prior art techniques, the Hart and Krause technique suffers from various disadvantages. One disadvantage involves the fact that the two halves of the mold have different material compositions, i.e., transparent plastic for the upper half and metal for the lower half. As is known in the art, it is in general easier to machine a material to mate with itself than to machine a material to mate with a different material. Also, the dimensional stability of a mold whose halves have different compositions is likely to be less than the dimensional stability of a mold whose halves have the same composition.
In addition to these problems with the apparatus used by Hart and Krause, the recoated fibers produced by their technique suffer a number of disadvantages. One particularly troublesome problem involves variations in the diameter of the recoated portion of the fiber caused by changes in the amount of resin shrinkage produced by different cure conditions. Also, after being removed from the mold, fibers recoated by the Hart and Krause procedure often include a relatively large flashing in the region of the injection port and, in some cases, include overflow of new resin onto the coated parts of the fiber adjacent to the splice because of the mismatch between the diameter of the mold and the diameter of the fiber.